JEC COMPOSITES MAGAZINE - Issue #67 - August/September 2011 - 45

blistering resistance is vital for marine applications. Blister resistance is a measure of osmosis and how quickly the gelcoat and laminate take up water. This blister performance test was originally developed by Scott Bader following significant work on GRP blistering in the early 1980’s. It has been used successfully for over 30 years to ensure gelcoats can be used below the water line with no surface degradation, when used as part of a matched marine laminate system. For this osmotic blister test, circular gelcoated test panels undergo rigorous blister testing (Figure 6), being permanently immersed in de-ionised water at 40°C for 12 months.
Fig.6: Circular gelcoat panels are placed in the blister test container and then undergo 12 months of testing

water uptake in grams over the 12 months by Crystic Permabright compared to the best-in-class Iso/NPG and Iso gelcoats. The water uptake results clearly show that Crystic Permabright has excellent long-term blister resistance. Based on these and other test results, Scott Bader has a high degree of confidence to approve the new gelcoat for use in white hulls below the water line, provided it is used as part of their matched Crystic marine laminate system with their VE 679PA skincoat and a marine-approved Crystic back-up resin.

Fig.8: Low shear viscosity variation over 3 months

Rheological properties
For ease of use on the shop floor during production, the importance of having optimised and consistent rheological profiles for the spray and brush grades has been taken into account by Scott Bader. The spray grade has been designed with a low viscosity at high shear rates to allow for ease of spraying, while having a high viscosity at low shear rates to prevent drainage and sagging. The rheological profile of Crystic gelcoats over time is closely monitored to ensure quality. The graphs (Figures 8 and 9) show their limited changes in viscosity over time at both high and low shear rates.

Fig.9: High shear viscosity variation over 3 months

Comparative gelcoat technology blister testing was also carried out. This critical performance requirement has also been successfully achieved by Crystic Permabright. The graph below (Figure 7) shows the comparative mean

Another important area which helps composite processors to ensure consistency in production is to use gelcoats and resins with a minimal geltime drift over time. Maintaining a consistent geltime has been incorporated into the overall formulation of Crystic Permabright which, as the graph in Figure 10 shows, exhibits a minimal geltime drift at 25°C of less than 4 Fig.7: Blister test results, measured by mean weight of minutes over a threewater uptake in grams during 12-month immersion in month period. de-ionised water at 40°C

Geltime drift

No67 August - September 2011 / jec composites magazine

Fig.10: Crystic Permabright has a minimal geltime drift over 3 months of less than 4 minutes

Application and reparability trials

The handling properties of both the spray- and brush-applied versions of Crystic Permabright are very similar to a conventional Iso/NPG gelcoat. It

Feature marine

JEC COMPOSITES MAGAZINE - Issue #67 - August/September 2011

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #67 - August/September 2011